The present invention relates in general to an implantable cardiac stimulation device capable of performing automatic capture. More specifically, the present invention is directed to a cardiac stimulation system and associated method for acquiring, storing, and displaying an evoked response feature log and a fusion event counter.
In the normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system causing a depolarization known as an R-wave and the resulting ventricular chamber contractions.
Disruption of this natural pacemaking and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac stimulation devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or anti-arrhythmia therapies to the heart at a desired energy and rate. A cardiac stimulation device is electrically coupled to the heart by one or more leads possessing one or more electrodes in contact with the heart muscle tissue (myocardium). One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.
A stimulation pulse delivered to the myocardium must be of sufficient energy to depolarize the tissue, thereby causing a contraction, a condition commonly known as xe2x80x9ccapture.xe2x80x9d In early pacemakers, a fixed, high-energy pacing pulse was delivered to ensure capture. While this approach is straightforward, it quickly depletes battery energy and can result in patient discomfort due to extraneous stimulation of surrounding skeletal muscle tissue.
The capture xe2x80x9cthresholdxe2x80x9d is defined as the lowest stimulation pulse energy at which capture occurs. By stimulating the heart chambers at or just above threshold, comfortable and effective cardiac stimulation is provided without unnecessary depletion of battery energy. Threshold, however, is extremely variable from patient-to-patient due to variations in electrode systems used, electrode positioning, physiological and anatomical variations of the heart itself, and so on. Furthermore, threshold will vary over time within a patient as, for example, fibrotic encapsulation of the electrode occurs during the first few weeks after surgery. Fluctuations may even occur over the course of a day or with changes in medical therapy or disease state.
Hence, techniques for monitoring the cardiac activity following delivery of a stimulation pulse have been incorporated in modern pacemakers in order to verify that capture has indeed occurred. If a loss of capture is detected by such xe2x80x9ccapture-verificationxe2x80x9d algorithms, the output is automatically increased until capture is restored. A threshold test is then performed by the cardiac stimulation device in order to re-determine the threshold and automatically adjust the stimulating pulse output.
This approach, referred to as xe2x80x9cautomatic capturexe2x80x9d, improves the cardiac stimulation device performance in at least four ways: 1) by verifying that the stimulation pulse delivered to the patient""s heart has been effective, 2) by maintaining the stimulation pulse output at the lowest level possible thus 3) greatly increasing the device""s battery longevity by conserving the energy used to generate stimulation pulses yet 4) always protecting the patient by providing a significantly higher output back-up pulse in the setting of loss of capture associated with the primary pulse.
One implemented technique for verifying capture automatically by an implantable stimulation device involves monitoring the internal myocardial electrogram (EGM) signal received on the cardiac stimulation and sensing electrodes. When a stimulation pulse is delivered to the heart, the EGM signals that are manifest concurrent with depolarization of the myocardium are examined. When capture occurs, an xe2x80x9cevoked responsexe2x80x9d may be detected by special evoked response detection circuitry. The evoked response is the intracardiac atrial or ventricular depolarization that is observed as the P-wave or R-wave, respectively, on the surface ECG. Detection of an evoked response indicates electrical activation of the respective cardiac tissue by the stimulating pulse. The depolarization of the heart tissue in response to the heart""s natural pacemaking function is referred to as an xe2x80x9cintrinsic responsexe2x80x9d.
Through sampling and signal processing algorithms, the presence of an evoked response following a stimulation pulse is determined. A very short blanking period, or period of absolute refractoriness, following the stimulation pulse is applied to the evoked response sensing circuit immediately following the stimulation pulse to minimize or block out the stimulation pulse artifact. This blanking period is followed by a special evoked response detection window, commonly 15 to 60 ms in duration, wherein the evoked response sensing circuit looks for an evoked response. For example, if a stimulation pulse is applied to the ventricle, an R-wave sensed by a special evoked response detection circuit of the pacemaker immediately following application of the ventricular stimulation pulse evidences capture of the ventricles.
If no evoked response is detected, a high-energy back-up stimulation pulse is delivered to the heart very shortly after the primary ineffective stimulus, typically within 60-100 ms of the primary pulse, in order to maintain the desired heart rate. If the loss of capture is sustained for more than one cardiac cycle, an automatic threshold test may be invoked in order to re-determine the minimum pulse energy required to capture the heart. Threshold tests may also be performed on a periodic basis, for example daily or weekly. An exemplary automatic threshold determination procedure is performed by first increasing the stimulation pulse output level to a relatively high predetermined testing level at which capture is certain to occur. Thereafter, the output level is progressively decremented until capture is lost. The stimulation pulse energy is then set to a level safely above the lowest output level at which capture was attained. Thus, reliable capture verification is of utmost importance in proper determination of the threshold.
One signal that interferes with the detection of an evoked response, and potentially the most difficult for which to compensate because it is usually present in varying degrees, is lead polarization. A lead-tissue interface is that point at which an electrode of the pacemaker lead contacts the cardiac tissue. Lead polarization is commonly caused by electrochemical reactions that occur at the lead-tissue interface due to application of an electrical stimulation pulse across the interface. If the evoked response is sensed through the same lead electrodes through which the stimulation pulses are delivered, the resulting polarization signal, also referred to as an xe2x80x9cafterpotentialxe2x80x9d, formed at the electrode can corrupt the evoked response signal that is sensed by the sensing circuits. This undesirable situation occurs often because the polarization signal can be three or more orders of magnitude greater than the evoked response. Furthermore, the lead polarization signal is not easily characterized; it is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy and other variables.
In order to verify that an evoked response is readily recognized during automatic capture verification or threshold testing, calibration methods are employed for measuring a characteristic of the post-stimulation signal, such as peak amplitude, slope, or the signal integral. Capture detection criteria can be determined by measuring the magnitude of a selected signal characteristic, or xe2x80x9csignal feature,xe2x80x9d during an evoked response and during pure polarization associated with non-capture.
Therefore, the post-stimulation signal after delivery of a stimulation pulse known to effectively capture the heart is evaluated to determine an evoked response signal feature. The post-stimulation signal after delivery of a stimulation pulse known not to capture the heart, e.g., by delivering the pulse during myocardial refractory when capture is impossible, is also evaluated. The difference between the evoked response signal features and the polarization signal features, is used to verify the presence of capture.
However, although the variability of the evoked response signal has been acknowledged, there is currently no way of ascertaining that the evoked response signal characteristic, upon which capture detection is based, does not change over time, regardless of stability in threshold.
Although calibration methods may be repeated in order to update the threshold detection criteria, it may not be desirable to do so because the automatic capture algorithm may have to be suspended during calibration. Calibration normally is performed in the clinic while automatic capture is disabled and under the supervision of a clinician. The clinician monitoring the calibration process may verify that the correct signals are sensed during the calibration process and that automatic capture is functioning properly afterward.
Another difficulty encountered in detecting an evoked response during automatic capture verification is the incidence of xe2x80x9cfusion.xe2x80x9d Fusion occurs when a stimulation pulse is delivered nearly simultaneously with a late intrinsic depolarization that goes undetected. Both the stimulation pulse and the intrinsic depolarization may contribute to the overall depolarization. The EMG signal during a fusion event becomes distorted causing the P-wave or R-wave to go undetected by the capture verification routine. A backup stimulation pulse and threshold test may be invoked when not clinically necessary.
Similarly, a xe2x80x9cpseudo-fusionxe2x80x9d event occurs when a stimulation pulse immediately follows a late intrinsic depolarization. The stimulation pulse does not contribute to the depolarization of the tissue but does obscure the EMG signal causing the R-wave or P-wave to go undetected, again triggering a back-up stimulation pulse and a threshold test.
Methods for determining when fusion or pseudo-fusion is suspected as the cause of a loss of capture detection have been proposed. If fusion is suspected, corrective actions may be taken by adjusting stimulation timing parameters in a way that might avoid fusion in the future, for example by shortening the pacing interval. Knowing the frequency at which fusion events, or suspected fusion events, occur would be useful to a physician in selecting operating parameters related to stimulation timing and automatic capture detection or in improving the methods by which fusion is detected or avoided.
It would therefore be desirable to periodically re-evaluate, in a cardiac stimulation device possessing automatic capture verification methods, the evoked response signal features. A log of the evoked response signal features may then be stored in memory and available for later display or analysis. Examination of the variability of the evoked response signal features over time, as well as the polarization signal features associated with loss of capture, will be useful to a clinician and scientist in evaluating the performance of automatic capture and in fine tuning the automatic capture operating parameters in individual patients. Examination of the variability of the evoked response can also be valuable in monitoring the electrophysiologic status of the patient. It would also be desirable to monitor the frequency of fusion events. If a high frequency of fusion is found, the corrective actions for avoiding fusion could be altered to enhance fusion avoidance.
The present invention addresses these and other needs by providing a cardiac stimulation device equipped with a xe2x80x9cfeature log,xe2x80x9d wherein each time a threshold test is performed, evoked response signal features and loss of capture signal features are evaluated and stored. The evoked response signal features, accumulated over time, can then be displayed and examined to assess evoked response variability for the purposes of evaluating and improving automatic capture performance and for the purpose of monitoring a patient""s electrophysiological status. The cardiac stimulation device also provides a xe2x80x9cfusion counterxe2x80x9d that allows the frequency of fusion to be monitored. Adjustments to fusion avoidance mechanisms may be made in response to a high incidence of fusion.
The foregoing and other features and advantages of the present invention are achieved by providing a cardiac stimulation system that includes an implantable device to sense cardiac signals and deliver stimulation therapy, and an external device to communicate with the implantable device via a telemetry circuit to send and/or receive programmed operating parameters and acquired cardiac data.
The implantable device is equipped with cardiac data acquisition capabilities. A preferred embodiment of the stimulation device includes a control system for controlling the operation of the device and executing various test algorithms such as capture verification, threshold testing, and fusion detection. The stimulation device further includes a set of leads for receiving cardiac signals and for delivering atrial and ventricular stimulation pulses; a set of sensing circuits comprised of sense amplifiers for sensing and amplifying the cardiac signals; and pulse generators for generating atrial and ventricular stimulation pulses. In addition, the stimulation device includes memory for storing operational parameters for the control system, and for storing acquired data in the feature log and fusion counter.
The stimulation device also includes a telemetry circuit for communicating with an external device. The external device preferably can be a user interface, such as a keyboard, mouse, or touch screen; a control system for controlling the operation of functions or tests carried out by the external programmer; a memory for storing control programs, operational parameters, or data received from the implantable device; and/or a display apparatus such an LCD screen or a printer. The external programmer also includes a telemetry unit for transmitting data to and receiving data from the implanted stimulation device.
The cardiac stimulation device of the present invention is capable of performing capture verification and threshold testing with the added ability of determining a number of post-stimulation signal features and storing these features in memory as a vector. To this end, the stimulation device includes an algorithm for acquiring post-stimulation EMG signals occurring within a xe2x80x9cdetection windowxe2x80x9d applied during threshold testing. From the acquired signals, the algorithm determines one or more signal features. These features are stored in a multi-byte feature vector along with stimulation parameter information and the date and time at which the feature vector was collected.
In a preferred embodiment, two feature vectors are acquired for the first two evoked response signals occurring during a threshold test, and two more feature vectors are acquired for two loss of capture signals occurring during the threshold test. The feature vectors may be acquired during threshold testing performed in response to loss of capture, or during periodic threshold testing, when automatic capture is enabled. If automatic capture is disabled, the feature vectors may be acquired by performing a threshold test at scheduled intervals.
The stored feature vectors may be downloaded to the external programmer at any time by a clinician. The signal features may be displayed graphically over time so that the clinician can evaluate variability of the evoked response. The signal feature data may also be statistically analyzed to determine variability or other signal behavior. Statistical reports may also be displayed on the external programmer.
The fusion counter included in the present invention is implemented in conjunction with a fusion detection algorithm. The fusion detection algorithm is executed in response to a loss of capture detection when automatic capture is enabled. Each time the fusion detection algorithm determines that fusion is suspected to have caused the loss of capture detection, the fusion counter is incremented. The number of suspected fusion events is then available to the automatic algorithms of the stimulation device for the purpose of adjusting fusion avoidance mechanisms as necessary, or can be downloaded and displayed on an external device to be evaluated by a clinician. The clinician may then adjust programmable operating parameters as appropriate for improving fusion detection and avoidance.
Thus, the present invention provides a system and associated methods for acquiring, storing, and displaying evoked response signal features for the purposes of monitoring evoked response variability and evaluating the performance of automatic capture. The present invention further provides a method for acquiring, storing, and displaying the number of suspected fusion events for the purpose of improving fusion avoidance through either automatic modification to fusion avoidance mechanisms or by providing a clinician with diagnostic information helpful in selecting programmable operating parameters.